Can probably be due to regional laws or religion itself, who knows? Idk how much influence religion has in Nigeria but if that was the major factor, then I reckon there's only a specific number of casinos that they themselves specifically allowed to sell their services in the country. If it was regional, same thing maybe, they just allowed a specific number of casinos instead of allowing everything all at once. Might be because they want to better regulate it, or just that the casinos paid the country so as to prevent competition.

Can also include that some providers aren't allowed in the country. And with casinos sometimes having said providers as their key entertainment, they can't exactly compete with existing ones or fully be able to sell their services to the country with those kinds of limitations.